Can I crossbreed these plants ...?

Why would you want to cross these two species? Since Sphagnum is a moss it doesn't produce flowers. So there is a problem of how you would be able to fertilize the Kenaf with the Sphagnum or vice versa. The only way you could hybridize the two would be to introduce Sphagnum genes into the Kenaf. Even then you may not get anything usable.